<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title></title>
</head>
<body>
	<?php 
  $a =1;
  $b = 2;
  $c = $a +$b;
  echo "$a";
  echo "<hr>";
  echo $a ,$b ,$c;
  echo "<hr>";
  $age = 18;
  $result = isset($age);  //判断变量是否有存储数据
  echo $result;  //实际结果是“true” .但echo输出1
  echo "<br>";
  var_dump($result);//var_dump()可以输出一个变量的完整信息
  $result2 = isset($d);
  var_dump($result2);
  unset($a); //销毁变量
  $result3 = isset($a);
  var_dump($result3);
  //echo $a;  报错
?>
</body>
</html>
